The present invention relates to compositions that are useful as catalyst activators for olefin polymerizations. More particularly the present invention relates to such compositions that are particularly adapted for use in the coordination polymerization of unsaturated compounds having improved activation efficiency and performance. Such compositions are particularly advantageous for use in a polymerization process wherein catalyst, catalyst activator, and at least one polymerizable monomer are combined under polymerization conditions to form a polymeric product.
It is previously known in the art to activate Ziegler-Natta polymerization catalysts, particularly such catalysts comprising Group 3-10 metal complexes containing delocalized .pi.-bonded ligand groups, by the use of an activator. Generally in the absence of such an activator compound, also referred to as a cocatalyst, little or no polymerization activity is observed.
A class of suitable activators are Lewis acids, especially alumoxanes, which are generally believed to be oligomeric or polymeric alkylaluminoxy compounds, including cyclic oligomers. Examples of alumoxanes (also known as aluminoxanes) include methylalumoxane (MAO) made by hydrolysis of trimethylaluminum as well as modified methylalumoxane (MMAO), wherein a portion of the trimethylaluminum in the foregoing hydrolysis is replaced by a higher trialkylaluminum compound such as triisobutylaluminum. MMAO advantageously is more soluble in aliphatic solvents than is MAO.
Generally alumoxanes contain on average about 1.5 alkyl groups per aluminum atom, and are prepared by reaction of trialkylaluminum compounds or mixtures of compounds with water (Reddy et al, Prog. Poly. Sci., 1995, 20, 309-367). The resulting product is in fact a mixture of various substituted aluminum compounds including especially, trialkylaluminum compounds (resulting from incomplete reaction of the trialkylaluminum starting reagent or decomposition of the alumoxane). The amount of such free trialkylaluminum compound in the mixture generally varies from 1 to 50 percent by weight of the total product.
Although effective in forming an active olefin polymerization catalyst when combined with a variety of Group 3-10 metal complexes, especially Group 4 metal complexes, generally a large excess of alumoxane compared to metal complex, such as, molar ratios from 100:1 to 10,000:1, is required in order to produce adequate rates of polymerization. Unfortunately, the use of such large excesses of cocatalyst is expensive and also results in polymer having an elevated residual aluminum content as well as lower molecular weight This former factor may adversely affect polymer properties, especially clarity and dielectric constant, whereas the latter issue relates to poor polymer performance.
Other types of monomeric aryloxyaluminum and arylamidoaluminum complexes have been found to be useful in metallocene catalyst activator packages, particularly as water and oxygenate scavengers. Examples include diisobutyl-2,6-di-t-butyl-4-methylphenoxyaluminum (DIBAL-BOT) as described in WO 97/27228 and Japanese kokai, 09-17629, or diisobutylhexamethyidisilylazayl aluminum (DIBAL-NS) as described by Rosen et al in WO 98/03558. Typically in such formulations, the Lewis acid, especially tris(pentafluorophanyl)borane, is first contacted with a metal complex to prepare the catalytically activated derivative. Thereafter, this derivative is generally not subject to ligand transfer with the aluminum compound.
A different type of activator compound is a Bronsted acid salt capable of transferring a proton to form a cationic derivative or other catalytically active derivative of such Group 3-10 metal complex, cationic charge transferring compounds, or cationic oxidizing activators, referred to collectively hereinafter as cationic activators. Preferred cationic activators are ammonium, sulfonium, phosphonium, oxonium, ferrocenium, silver, lead, carbonium or silylium compounds containing a cation/anion pair that is capable of rendering the Group 3-10 metal complex catalytically active. Preferred anions associated with this cation comprise fluorinated arylborate anions, more preferably, the tetrakis(pentafluorophenyl)borate anion. Additional suitable anions include sterically shielded, bridged diboron anions. Examples of such cationic activators are disclosed in U.S. Pat. No. 5,198,401, U.S. Pat. No. 5,132,380, U.S. Pat. No. 5,470,927, U.S. Pat. No. 5,153,157, U.S. Pat. No. 5,350,723, U.S. Pat. No. 5,189,192, U.S. Pat. No. 5,626,087 and in U.S. Pat. No. 5,447,895.
Further suitable activators for activating metal complexes for olefin polymerization include neutral Lewis acids such as tris(perfluorophenyl)borane and tris(perfluorobiphenyl)borane. The former composition has been previously disclosed for the above stated end use in U.S. Pat. No. 5,721,185, and elsewhere, whereas the latter composition is disclosed in Marks, et al, J. Am. Chem. Soc. 1996, 118, 12451-12452. Additional teachings of the foregoing activators may be found in Chen, et al, J. Am. Chem. Soc. 1997, 119, 2582-2583, Jia et al, Organometallics, 1997, 16, 842-857. and Coles et al, J. Am. Chem. Soc. 1997, 119, 8126-8126.
Tris(perfluorophenyl)aluminum is a strong Lewis acid as well. It has recently been prepared from the exchange of tris(perfluorophenyl)borane with trialkylaluminum, which gives a trialkylborane and tris-perfluorophenylaluminum, as described by Biagini et al U.S. Pat. No. 5,602,269. However, it generally performs poorly by itself as a catalyst activator compared with tris(perfluorophenyl)borane when used in an equimolar ratio with a metal complex. Similarly, It has further been demonstrated that active catalysts resulting from the use of an aluminate anion based upon tris-(perfluorophenyl)aluminum for the activation of ansa-metallocenes and biscyclopentadienyl derivatives of zirconium(IV) are generally of lower activity than those formed by the corresponding borane (Ewen, Stud. in Surf. Sci. Catal. 1994, 89, 405-410). A possible explanation for the poor performance of tris(perfluorophenyl)aluminum as an activator for metallocenes involving a back exchange reaction of a perfluorophenyl group has been proposed by Bochmann et al (ACS Dallas Meeting, March 1998, Abs. number INOR 264, subsequently published, Organometallics, 1998, 17, 5908-5912).
In light of these apparent deficiencies, it would be desirable to provide novel compounds having improved efficiency and operability as activators of metal complexes for olefin polymerizations.